In recent years, a number of medical devices have been designed which are adapted for compression into a small size to facilitate introduction into a vascular passageway and which are subsequently expandable into contact with the walls of the passageway. Such devices have often been formed of shape memory material, such as nitinol, an alloy of titanium and nickel. Shape memory in such devices can be thermally triggered or stress induced.
In the thermally triggered devices, the device is cooled below a temperature transformation level to a martensitic state and thereby softened for loading into a catheter in a relatively compressed and elongated state. To regain the memorized shape in an austenitic state, the device is warmed to a selected temperature transformation level, such as human body temperature. The two interchangeable shapes are possible because of the two distinct microcrystalline structures that are interchangeable with a small variation in temperature. The temperature at which the device assumes its first configuration may be varied within wide limits by changing the composition of the alloy. Thus, for human use, the alloy may be focused on a transition temperature range close to 98.6xc2x0 F.
For stress induced shape memory as described by U.S. Pat. No. 4,665,906 to Jervis, stress is applied to a shape memory device which is in the austenitic state at room temperature. As the alloy forming the device is deformed elastically, it enters the martensitic state and can be loaded into a catheter or delivery tube. When the stress is removed, the alloy recovers elastically and reverts to austenite without requiring a change in temperature. The recovery always returns the material to the initial shape present in the austenitic phase before stress was applied.
The development of intra vascular medical devices such as stents and filters which expand and are held in position by engagement with the inner wall of a vessel has led to the development of intra vascular snares to retrieve these foreign bodies from the peripheral vessels of the cardiovascular system. Single loop snares, such as those shown by U.S. Pat. No. 3,828,790 to Curtiss et al. and U.S. Pat. No. 5,171,233 to Amplatz et al. are commonly used snares. The Amplatz snare consists of a super-elastic nitinol cable with a single-formed loop. Because of the snare""s super elastic construction, the loop can be introduced through small lumen catheters without risk of deformation. The loop is formed at approximately 90xc2x0 to the cable, and this allows for the user to advance the loop over a foreign body and ensnare it by closing the loop with a small catheter. The foreign body is removed from the vasculature by withdrawing the device into a guiding catheter or vascular sheath. Although this snare is a significant improvement over earlier forward facing stainless steel snares that are easily deformed and difficult to advance over foreign bodies, the Amplatz snare is geometrically sensitive and requires appropriate sizing to the vasculature in order to allow for successful ensnarement. In addition, the single loop design has poor cross sectional vessel coverage and thereby requires skilled manipulation to capture the desired object.
In an attempt to provide a snare with improved cross sectional vessel coverage, multi loop snares such as those shown by U.S. Pat. No. 5,098,440 to Hillstead and U.S. Pat. No. 6,099,534 to Bates have been developed. These snares include loops which are joined only at their proximal ends to a shaft, and otherwise are not joined at any point between the shaft and the distal ends of the loops. This provides the advantage over single loop. snares of enhanced cross sectional vessel coverage, and the free distal ends of the loops can be brought together to engage multiple surfaces of an intravascular medical device to be removed. The problem with multiloop snares having loops attached at only the proximal ends is that the relative geometry of the free loops is difficult to maintain. The relative position of the loops can change, both within a catheter or delivery tube and within a vessel, and the loops can actually become displaced or entangled during delivery.
Basket type snares having loops connected at both their proximal and distal ends to form an enclosed basket have been developed as shown by U.S. Pat. No. 5,817,104 to Bilitz et al. and U.S. Pat. No. 6,077,274 to Ouchi et al. These basket snares do not have free distal ends which can move along opposed surfaces of an intravascular device, and basket snares are primarily used to remove stones or stone fragments from the gallbladder, bilary tract, renal pelvis and ureter.
It is a primary object of the present invention to provide a novel and improved intravascular snare and method of forming the same wherein the snare includes multiple loops which are free at their distal ends and are joined at their proximal ends and at positions between the proximal and distal ends.
Another object of the present invention is to provide a novel and improved intravascular snare and method of forming the same wherein the snare includes a plurality of loops which are interlaced together between their proximal and distal ends to maintain a predetermined loop geometry while maintaining the distal ends of the loops free.
A further object of the present invention is to provide a novel and improved intravascular snare and method of forming the same wherein the snare includes a plurality of loops attached at their proximal ends to a central shaft. The loops angle outwardly relative to the shaft at approximately a ten to thirty degree angle and spaced from the shaft are angled outwardly at about another ten to thirty degrees to create a greater diameter at the loop distal ends. The total angle of each loop to the shaft is preferably forty degrees or less.
Yet another object of the present invention is to provide a novel and improved intravascular snare and method of forming the same wherein the snare includes multiple loops of a composite multiple strand material with the strands of each loop being penetrated by an adjacent loop to interlace the loops together.
A still further object of the present invention is to provide a novel and improved intravascular snare and method of making the same wherein the snare is formed of shape memory material such as nitinol. Martensite is induced in the material, such as by cooling, and in the martensitic state, the material is deformed mechanically to form a new shape by controlled deformation. If cycled elastically, the alloy will remember the new shape rather than the original austenitic shape.
These and other objects of the present invention are achieved by forming a snare comprised of a central shaft constructed of super-elastic nitinol with two to eight pre-formed interlaced loops at the distal end of the shaft. The loops are formed of equal length and preferably extend at approximately 15xc2x0 to the central shaft and each loop is also preferably flared outwardly an additional 15xc2x0 to open the leading edges of the loops to a greater diameter that ultimately increases wire to vessel surface area contact. Individual loops are interlaced together to form a tulip shaped assembly. The loops are fonxed from a composite multiple strand material constructed of nitinol and a noble metal such as gold or platinum iridium. The noble metal provides radiopacity while the nitinol provides shape memory.